A so-called polymer electrolyte fuel cell (Polymer Electrolyte Fuel Cell: hereinafter called “PEFC” as needed), has its operating temperature of from a room temperature to about 80° C. Also, since PEFC makes it possible to employ inexpensive general-purpose plastics, etc. for members constituting its fuel cell body, it is possible to realize reduction in weight. Furthermore, PEFC makes it possible to achieve thinning of a polymer electrolyte membrane, enabling an electric resistance to be reduced, thereby enabling a power loss to be reduced relatively easily. Due to PEFC having not a few advantages as described above, it is applicable to a fuel cell vehicle, a home cogeneration system, and the like.
As an electrode catalyst for PEFC, there has been proposed an electrode catalyst in which a platinum (Pt) or platinum (Pt) alloy, i.e., a component for the electrode catalyst, is supported on a carbon serving as a support (for example, Non-Patent Document 1).
Conventionally, there have been disclosed that, as for an electrode catalyst for PEFC, if the content of chlorine contained in the electrode catalyst is 100 ppm or more, it is not desirable as an electrode catalyst (for example, Patent Document 3); and that this is because if the content of chlorine contained in the electrode catalyst is 100 ppm or more, it is impossible to obtain a sufficient catalytic activity for the electrode catalyst for fuel cells; and corrosion of its catalyst layer will occur, thus shortening the life of the fuel cell.
Then, there is proposed a PEFC equipped with a membrane electrode assembly with electrodes joined on both sides of an electrolyte membrane, in which acid radical protons contained in an inner-catalyst-layer electrolyte are partially exchanged by phosphonium ion (for example, Patent Document 1). In the PEFC, the counter anion of the phosphonium ion is a compound containing no halogen elements. The reason, as disclosed therein, is because a halogen element causes degradation in battery performance if it remains in the electrode.
Also, there is proposed a PEFC equipped with a membrane electrode assembly (MEA) with electrodes including a catalyst layer joined on both sides of an electrolyte membrane, (for example, Patent Document 2). Pd and Pt as a catalyst component of the Pd/Pt particles contained in the catalyst layer of the PEFC are derived from a halide.
Thus, the method of manufacturing the Pd/Pt particles described in Patent Document 2 employs a halogen-free compound for an ion-exchange liquid used in preparing Pd and Pt. Patent Document 2 discloses that the use of halogen should be avoided because halogen ion degrades battery performance. Patent Document 2 discloses warm water cleaning as a method for deionization treatment with respect to halogen.
Then, there is disclosed a method for preparation of a powder of platinum (Pt) or platinum (Pt) alloy that contains less than 100 ppm of chlorine as the catalyst component of the electrode catalyst (for example, Patent Document 3).
As for the preparation of a powder of the platinum (Pt) or platinum (Pt) alloy, there is disclosed the following method: forming a melt which contains a low-melting mixture of alkali-metal nitrate, a chlorine-free platinum compound and a chlorine-free compound of alloying elements; heating the melt up to a reaction temperature at which the platinum compound and the compound of the alloying elements are thermally decomposed to give an oxide; cooling the melt; and the melt is dissolved in water and the resulting oxide or mixed oxides are converted into a powder of platinum or platinum alloy by successive reduction.
Whereas, it becomes important in the future development of PEFC, to pursue reduction of cost in a variety of ways, while maintaining or improving power generation performance toward the practical use thereof.
For this reason, study from the same point of view becomes so important in the development of electrode catalyst as well that there has been conducted the study of an electrode catalyst having a so-called core-shell structure (core-shell catalyst) (for example, Patent Document 4, Patent Document 5).In manufacturing processes of such core-shell catalyst, a metal chloride salt is often used as a raw material.
For example, Patent Document 4 and Patent Document 5 disclose a core-shell catalyst employing palladium as a constituent element of a core part and platinum as a constituent element of a shell part, showing, as one example of a raw material for such shell part, potassium chloroplatinate.
For such core-shell catalyst employing palladium as a constituent element of the core part, and platinum as a constituent element of the shell part, there are often employed, as a raw material, materials containing chloride (Cl) species such as platinum (Pt) chloride salt, palladium (Pd) chloride salt. This is presumably due to the fact that the chloride salts of platinum (Pt) and palladium (Pd) are easily available, and easy to use under their manufacturing conditions, resulting in comparatively low cost of raw materials. For this reason, it is difficult to respond to the need to let the core-shell catalyst exhibit sufficient catalytic activity, by positively choosing, as a starting material, a compound containing no halogen elements (particularly chlorine).
Meanwhile, the present applicant submits, as publications where the above-mentioned publicly-known inventions are described, the following publications: